1. Field of the Invention
The invention relates in general to a method and an apparatus for improving gray-scale linearity, and more particular, to a method and an apparatus for improving gray-scale linearity of a plasma display.
2. Related Art of the Invention
The earliest dynamic image that the human beings are people were able to see is was the documentary film movie. Later on, with the invention of cathode ray tube (CRT) successfully derives the came commercialized televisions, which then become the an essential appliance for every family. The cathodes ray tubes are became further applied to as the desktop monitor in computers industries for several decades. However, due to the irresolvable radiation problems and the very large volume occupied by the electron gun, the very large displays made thereof gradually have fallen behind the trends of being thin, light and of large-area.
To address the above problems, flat panel displays have been developed. The currently developed flat panel displays include liquid crystal display (LCD), field emission display (FED), vacuum fluorescent display (VFD), organic light emitting diode (OLED) and plasma display panel (PDP). Among these flat panel displays, the plasma display panel is often time applied to digital television and has great market potential due to the advantages of high resolution, high image quality, and large display area.
The generation of plasma display panel is to improve the drawbacks of cathode ray tube. It is easy for to have the electron beam to approach the screen corner by simply deflecting the electron beam with by a large angle. However, the large deflection angle results in an excessively large spot, such that the image and picture are distorted, and the resolution around the screen corners of the cathode ray tube is poor. When the internal space of the cathode ray tube is continuously expanded, it is difficult to maintain high vacuum. Therefore, fabrication of large screen is difficult. In addition, the cathode ray tube does not have the memory functions. To resolve theses drawbacks, Dr.″s D. L. Bitzer and H. G. Slottow have developed the plasma display panels.
With respect to the basic structure, the plasma display panel can be classified into DC type and AC type displays. The display theory includes applying a voltage to a cell where the X-axial electrode and the Y-axial electrode intersects. When the voltage approaches to a certain level (such as 180V), the gas atom is electrically ionized. The energy level of the inert gas is thus enhanced. When the inert gas atom returns from the high energy level to the ground energy level, an ultraviolet light is generated. The fluorescent material coated in the discharge space is then excited by the ultra-violet light to emit visible light within a specific frequency. The structures for DC plasma display panel and the AC plasma display panel are very similar. For example, the intersections between the X-axial electrodes and the Y-axial electrodes are the space for discharge luminescence. The difference is that the electrode of the AC plasma display panel is coated with a dielectric layer (such as MgO), while the electrode of the DC plasma display panel does not include such a layer. Therefore, the X-axial electrodes and the Y-axial electrodes are directly exposed in the discharge space, such that electrons and ions are induced on the wall to result in the memory function.
The plasma display panel can displays various illuminations by through sustain pulse control. With regards to the method of gray scale display for the AC plasma display panel, the sustain pulse period of a field (typically 1/60 sec.) is distributed into several sub-fields (SF), where each sub-field has several a different sustain pulseperiod. By using different combinations of the sub-fields, different gray scales are displayed. To give a clearer picture, referring to FIG. 1, the brightness weight of the sub-field of an AC plasma display panel is illustrated. In FIG. 1, a field is divided into 9 sub-fields SF0 to SF8. Each sub-field includes a constant address period 102 and a different sustain period 104 according to the number of the sustain pulses. The more the sustain pulses are, the longer the sustain period 104 lasts. Assuming that 8 bits are used to represent the gray scale of the plasma display panel, there are 256 gray scales, 0-255, to be represented. Assuming that the gray scale 1 is corresponding corresponds to 20 sustain pulses, the number of sustain pulses for the sub-field SF0 is thus 20. If the number of sustain pulses for the sub-fields SF1 to SF8 are 40, 80, 140,260, 520, 920, 1360 and 1800, respectively, the sub-fields SF1 to SF8 represents the gray scales 2, 4, 7, 1 3, 26, 46, 68 and 90, respectively. Other gray scales can be assembled by allocation of different sub-fields. For example, the gray scale 5 can be assembled by the sub-fields SF0 and SF2, and the gray scale 15 can be represented by the combination of the sub-fields SF1 and SF4. Generally speaking, a simple linear relationship between the allocated gray scale and the brightness is expected. If, As as shown in FIG. 1, if the weight ratio of the number of the sustain pulses for each sub-field is SF0:SF1:SF2:SF3:SF4:SF5:SF6:SF7:SF8=1:2:4:7:13:26:46:68:90, two problems occur:
(1) The brightness ratio for each sub-field will not be 1:2:4:7:13:26:46:90; and
(2) Even with the brightness ratio of 1:2:4:7:1 3:26:46:90, the brightness after combination is lower than the sum of each sub-field.
The above two problems seriously affect the linearity between gray scale and brightness, so as to affect the display quality.
To improve the gray scale linearity of the plasma display panel, in U.S. Pat. No. 5,943,032, Japanese manufacturer Fujitsu disclosed a method for changing number of sustain pulses. In this method, the number of sustain pulses for each sub-field is adjusted to improve the linearity for gray-scale versus brightness. For example, when the number of sustain pulses of gray scale 4 is 60, and the measured brightness of the gray scale 4 is lower than 60 cd/m2, the number of sustain pulses for the gray scale 4 is increased to 80, such that the brightness is increased to 60 cd/m2 to improve the linearity for gray scale versus brightness. However, by applying such method to the example as shown in FIG. 1, as 9 fields require only 9 kinds of sustain pulses, only 9 parameters are provided for adjustment, so that the gray scale linearity for all gray scales cannot be improved.
Another method to improve the gray scale linearity of the plasma display panel is disclosed in U.S. Pat. No. 5,943,032 by Korean Manufacturer LG. Such method employs image distortion compensation unit to add a pseudo pulse in the sustain pulse region of the sub-field to increase the brightness of the sub-field, so as to improve the linearity of gray scale versus brightness. Similarly, by applying this method to the example in FIG. 1, only 9 sustain pulses are provided by 9 sub-fields. Therefore, only 9 parameters are provided for the added pseudo pulse for adjustment. One cannot improve the gray scale linearity for all gray scales.